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fmt trunc fill character into 16bit, even for wchar_t(linux) or char32_t #4201

@XZiar

Description

@XZiar

In UTF32, a code point is 4 bytes, but fmt only keeps 2 bytes when parsing it into fill:

fmt/include/fmt/base.h

Lines 800 to 825 in cc2ba8f

template <typename Char> constexpr auto fill_unit() const -> Char {
using uchar = unsigned char;
return static_cast<Char>(static_cast<uchar>(fill_data_[0]) |
(static_cast<uchar>(fill_data_[1]) << 8));
}
FMT_CONSTEXPR void set_fill(char c) {
fill_data_[0] = c;
set_fill_size(1);
}
template <typename Char>
FMT_CONSTEXPR void set_fill(basic_string_view<Char> s) {
auto size = s.size();
set_fill_size(size);
if (size == 1) {
unsigned uchar = static_cast<detail::unsigned_char<Char>>(s[0]);
fill_data_[0] = static_cast<char>(uchar);
fill_data_[1] = static_cast<char>(uchar >> 8);
return;
}
FMT_ASSERT(size <= max_fill_size, "invalid fill");
for (size_t i = 0; i < size; ++i)
fill_data_[i & 3] = static_cast<char>(s[i]);
}
};

meanwhile, UTF8 can keep 4 bytes for fill.

repro: https://godbolt.org/z/qfTKe6W6c

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